A latex turbidimetric immunoassay (hereinafter also referred to as LTIA) is a method of measuring a test substance (analyte) by using latex particles to which antigens or antibodies are immobilized (antigen- or antibody-immobilized latex particles), and is widely used in the field of clinical examination. Methods of measuring an antigen which is an analyte (analyte antigen) by LTIA are roughly classified into a method of forming a sandwich type immune complex through a reaction between an anti-analyte antibody-immobilized latex particle and an analyte antigen so as to measure the analyte (antigen) from the level of agglutination of the latex particles associated with the formation of an immune complex (hereinafter also referred to as sandwich LTIA), and a method of causing an antigen-immobilized latex particle and an antigen (analyte) in a test sample to compete with each other so that the formation of an immune complex is inhibited between the latex particle and the antibody so as to measure the analyte (antigen) from the level of agglutination inhibition of the latex particles associated with the inhibition of formation of the immune complex (hereinafter also referred to as agglutination inhibition LTIA).
In LTIA, it is commonplace that an agglutination of antigen- or antibody-immobilized latex particles that should not occur does occur due to some kind of component in a test sample even through an analyte is not present in the test sample such as serum, conversely, it is also common that an agglutination that should occur does not occur. This is referred to as a nonspecific reaction and is known to cause various measurement errors.
A known method for avoiding nonspecific reactions is a technique in which various substances are added to a reaction system.
Patent Document 1 describes a method in which an inorganic boron compound is added to a sample solution in combination with a buffer system as a method for removing a nonspecific turbidity (synonymous with nonspecific agglutination) in the sandwich LTIA. However, this method is a method of removing the occurrence of agglutination that should not occur in an assay using antibody-immobilized latex particles and is not a method capable of dealing with the absence of agglutination that should occur in agglutination inhibition LTIA.
In the case of measurement of a blood sample by agglutination inhibition LTIA, even if the blood sample does not contain an analyte, only a lower degree of agglutination is in some cases acquired than the degree of agglutination acquired when measurement is performed by using as a test sample a buffer solution containing no blood component, due to an effect of some kind of component in the blood sample. Alternatively, the degree of agglutination (absorbance) is in some cases not identical and different between the cases of diluting, with a buffer solution and with a buffer solution containing a blood component, a standard substance for concentration calibration of an analyte in a test sample (hereinafter also referred to as a concentration calibration-standard substance). However, as described above, no method has been established for avoiding nonspecific reactions which disrupt the agglutination that should occur in agglutination inhibition LTIA even in the present time and a new method needs to be developed.